PHA-rubber blends: synthesis, characterization and biodegradation

Comparing environmental fate and ecotoxicity of conventional and biodegradable plastics: A critical review

Plastic pollution is a consequential problem worldwide, prompting the widespread use of biodegradable plastics (BPs). However, not all BPs are completely degradable under natural conditions, but instead produce biodegradable microplastics (BMPs), release chemical additives, and absorb micropollutants, thus causing toxicity to living organisms in similar manners to conventional plastics (CPs). The new problems caused by biodegradable plastics cannot be ignored and requires a thorough comparison of the differences between conventional and biodegradable plastics and microplastics. This review comprehensively compares their environmental fates, such as biodegradation and micropollutant sorption, and ecotoxicity in soil and water environments. The results showed that it is difficult to determine the natural conditions required for the complete biodegradation of BPs. Some chemical additives in BPs differ from those in CPs and may pose new threats to ecosystems. Because of functional group differences, most BMPs had higher micropollutant sorption capacities than conventional microplastics (CMPs). The ecotoxicity comparison showed that BMPs had similar or even greater adverse effects than CMPs. This review highlights several knowledge gaps in this new field and suggests directions for future studies.

Avocado seed waste bioconversion into poly(3-hydroxybutyrate) by using Cobetia amphilecti and ethyl levulinate as a green extractant

The avocado processing industry produces up to 1.3M tons of agro-waste annually. Chemical analysis of avocado seed waste (ASW) revealed that it is rich in carbohydrates (464.7 ± 21.4 g kg^-1) and proteins (37.2 ± 1.5 g kg^-1). Optimized microbial cultivation of Cobetia amphilecti using an acid hydrolysate of ASW, generated poly(3-hydroxybutyrate) (PHB) in a 2.1 ± 0.1 g L^-1 concentration. The PHB productivity of C. amphilecti cultivated on ASW extract was 17.5 mg L^-1 h^-1. The process in which a novel ASW substrate was utilized has been further augmented by using ethyl levulinate as a sustainable extractant. This process achieved 97.4 ± 1.9 % recovery yield and 100 ± 1 % purity (measured by TGA, NMR, and FTIR) of the target PHB biopolymer, along with a high and relatively uniform PHB molecular weight (M_w = 1831 kDa, M_n = 1481 kDa, M_w/M_n = 1.24) (measured by gel permeation chromatography), compared to PHB polymer extracted by chloroform (M_w = 389 kDa, M_n = 297 kDa, M_w/M_n = 1.31). This is the first example of ASW utilization as a sustainable and inexpensive substrate for PHB biosynthesis and ethyl levulinate as an efficient and green extractant of PHB from a single bacterial biomass.

Enhancing impact resistance and biodegradability of PHBV by melt blending with ENR

This research aims to enhance the mechanical characteristics of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) by using epoxidized natural rubber (ENR-25 and ENR-50) as a toughening agent and polybutadiene (PB) grafted with maleic anhydride (MA) (3 MA groups/chain) as a compatibilizer. The PHBV/ENR blends were mixed in 100/0, 90/10, 80/20, and 70/30 with PB-g-MA at 0, 5, and 10% (wt./wt.), using an internal mixer set to 175 °C with a rotor speed of 50 rpm. The findings indicated that at 70/30 PHBV/ENR composition, the impact strength of the blends with 25 and 50 epoxide contents were the greatest at 6.92 ± 0.35 J m^-1 and 7.33 ± 1.19 J m^-1, respectively, which are about two times greater than that of neat PHBV. Furthermore, the biodegradability of the PHBV/ENR blends was more substantial than that of neat PHBV, showing a mass reduction of approximately 40% and 45% for PHBV/ENR-25 and PHBV/ENR-50, respectively. In comparison, while the mass loss of PHBV was approximately 37% after three months of soil burial. The results indicate that ENR improves the toughness of the blends while simultaneously increasing PHBV degradation, which could pave the way for broadening PHBV for sustainability purposes.

Polyhydroxyalkanoate bio-production and its rise as biomaterial of the future

The first observation of a polyhydroxyalkanoate (PHA) aggregate was in 1888 by Beijenrinck. Despite polyhydroxybutyrate (PHB) being the first type of PHA discovered, it was not extracted and characterized until 1925 by Maurice Lemoigne in France, even before the concept of "macromolecules" was known. After more than 30 years, in 1958, Wilkinson and co-workers rediscovered PHB and its metabolic role in the cells as storage compound. PHB started to be appealing to the industry in the 1980s, when a few companies started to commercialize microbially produced PHAs. During the 1990 s, the focus was on reducing production costs to make PHA production economically feasible, for instance by genetically modified microorganisms and even plants. Since then, many advances have been made: diverse wastes as feedstock, different production processes, and tailored design of biopolymers. This paper summarizes the scientific and technological development of PHAs from their discovery in 1888 until their latest applications and current commercial uses. Future perspectives have been devised too based on the current bottlenecks.

A Review on Biological Synthesis of the Biodegradable Polymers Polyhydroxyalkanoates and the Development of Multiple Applications

Polyhydroxyalkanoates, or PHAs, belong to a class of biopolyesters where the biodegradable PHA polymer is accumulated by microorganisms as intracellular granules known as carbonosomes. Microorganisms can accumulate PHA using a wide variety of substrates under specific inorganic nutrient limiting conditions, with many of the carbon-containing substrates coming from waste or low-value sources. PHAs are universally thermoplastic, with PHB and PHB copolymers having similar characteristics to conventional fossil-based polymers such as polypropylene. PHA properties are dependent on the composition of its monomers, meaning PHAs can have a diverse range of properties and, thus, functionalities within this biopolyester family. This diversity in functionality results in a wide array of applications in sectors such as food-packaging and biomedical industries. In order for PHAs to compete with the conventional plastic industry in terms of applications and economics, the scale of PHA production needs to grow from its current low base. Similar to all new polymers, PHAs need continuous technological developments in their production and material science developments to grow their market opportunities. The setup of end-of-life management (biodegradability, recyclability) system infrastructure is also critical to ensure that PHA and other biobased biodegradable polymers can be marketed with maximum benefits to society. The biobased nature and the biodegradability of PHAs mean they can be a key polymer in the materials sector of the future. The worldwide scale of plastic waste pollution demands a reformation of the current polymer industry, or humankind will face the consequences of having plastic in every step of the food chain and beyond. This review will discuss the aforementioned points in more detail, hoping to provide information that sheds light on how PHAs can be polymers of the future.

Microalgae as Contributors to Produce Biopolymers

Biopolymers are very favorable materials produced by living organisms, with interesting properties such as biodegradability, renewability, and biocompatibility. Biopolymers have been recently considered to compete with fossil-based polymeric materials, which rase several environmental concerns. Biobased plastics are receiving growing interest for many applications including electronics, medical devices, food packaging, and energy. Biopolymers can be produced from biological sources such as plants, animals, agricultural wastes, and microbes. Studies suggest that microalgae and cyanobacteria are two of the promising sources of polyhydroxyalkanoates (PHAs), cellulose, carbohydrates (particularly starch), and proteins, as the major components of microalgae (and of certain cyanobacteria) for producing bioplastics. This review aims to summarize the potential of microalgal PHAs, polysaccharides, and proteins for bioplastic production. The findings of this review give insight into current knowledge and future direction in microalgal-based bioplastic production considering a circular economy approach. The current review is divided into three main topics, namely (i) the analysis of the main types and properties of bioplastic monomers, blends, and composites; (ii) the cultivation process to optimize the microalgae growth and accumulation of important biobased compounds to produce bioplastics; and (iii) a critical analysis of the future perspectives on the field.

Green Natural Rubber Composites Reinforced with Black/White Rice Husk Ashes: Effects of Reinforcing Agent on Film's Mechanical and Dielectric Properties

Green natural rubber (NR) composites reinforced with black rice husk ash (BRHA)/white rice husk ash (WRHA), using alginate as a thickening and dispersing agent and crosslinking by CaCl₂, was developed to improve mechanical, chemical and dielectric properties of NR-based films by using a latex aqueous microdispersion process. A maximum of 100 per hundred rubbers (phr) of rice husk ashes (RHAs) could be integrated in NR matrix without phase separation. Mechanical properties of the composite films were considerably enhanced, compared to the neat NR film. The composite films reinforced with WRHA demonstrated relatively better mechanical properties than those reinforced with BRHA, whereas the composites filled with BRHA demonstrated higher elongation at break. The crosslinking by CaCl₂ improved the film tensile strength but lowered the film elasticity. The reinforcement strongly improved chemical resistance of the composite films in toluene. The films are biodegradable in soil, with weight loss of 7.6–18.3% of the initial dry weight after 3 months. Dielectric constant and dielectric loss factors of the composite films were enhanced with RHAs loading. According to the obtained properties, the composites offer potential for further development as stretchable conductive substrate or semiconducting polymer films for electronic applications.

Biodegradation of Wasted Bioplastics in Natural and Industrial Environments: A Review

The problems linked to plastic wastes have led to the development of biodegradable plastics. More specifically, biodegradable bioplastics are the polymers that are mineralized into carbon dioxide, methane, water, inorganic compounds, or biomass through the enzymatic action of specific microorganisms. They could, therefore, be a suitable and environmentally friendly substitute to conventional petrochemical plastics. The physico-chemical structure of the biopolymers, the environmental conditions, as well as the microbial populations to which the bioplastics are exposed to are the most influential factors to biodegradation. This process can occur in both natural and industrial environments, in aerobic and anaerobic conditions, with the latter being the least researched. The examined aerobic environments include compost, soil, and some aquatic environments, whereas the anaerobic environments include anaerobic digestion plants and a few aquatic habitats. This review investigates both the extent and the biodegradation rates under different environments and explores the state-of-the-art knowledge of the environmental and biological factors involved in biodegradation. Moreover, the review demonstrates the need for more research on the long-term fate of bioplastics under natural and industrial (engineered) environments. However, bioplastics cannot be considered a panacea when dealing with the elimination of plastic pollution.

Methodologies to assess biodegradation of bioplastics during aerobic composting and anaerobic digestion: A review

Bioplastics are emerging on the market as sustainable materials which rise to the challenge to improve the lifecycle of plastics from the perspective of the circular economy. The article aims at providing a critical insight of research studies carried out in the last 20 years on the degradation of bioplastics under aerobic composting and anaerobic digestion conditions. It mainly focuses on the various and different methodologies which have been proposed and developed to monitor the process of biodegradation of several bioplastic materials: CO₂ and CH₄ measurements, mass loss and disintegration degree, spectroscopy, visual analysis and scanning electron microscopy. Moreover, across the wide range of studies, the process conditions of the experimental setup, such as temperature, test duration and waste composition, often vary from author to author and in accordance with the international standard followed for the test. The different approaches, in terms of process conditions and monitoring methodologies, are pointed out in the review and highlighted to find significant correlations between the results obtained and the experimental procedures. These observed correlations allow critical considerations to be reached about the efficiency of the methodologies and the influence of the main abiotic factors on the process of biodegradation of bioplastics.

Synthesis of Short-Chain-Length and Medium-Chain-Length Polyhydroxyalkanoate Blends from Activated Sludge by Manipulating Octanoic Acid and Nonanoic Acid as Carbon Sources

The effects of octanoic acid/nonanoic acid and acclimation time on the synthesis of short-chain-length and medium-chain-length PHA blends from activated sludge were investigated. An increased concentration (847-1366 mg/L) of PHAs resulted from 4-month acclimation compared with the concentration derived from 2-month acclimation (450-1126 mg/L). The content of octanoic acid had a positive linear relationship with the content of even-numbered carbon monomers among the PHAs. The blending products were identified mainly with scl-PHAs during the 2-month acclimation period and were thereafter dominated by mcl-PHAs until 4 months of acclimation. Thermal properties analysis demonstrated that the products derived from 4-month acclimation were a mixture of scl-PHAs and mcl-PHAs rather than a copolymer of scl-PHAs and mcl-PHAs. High-throughput sequencing results indicated that Pseudofulvimonas, Paracoccus, and Blastocatella were the dominant genera that might be responsible for scl-PHAs production during the 2-month acclimation period, whereas Comamonas and Pseudomonas that were responsible for mcl-PHAs production then became the dominant genera after 4-months acclimation.

Biodegradability of carbon nanotube/polymer nanocomposites under aerobic mixed culture conditions

The properties and commercial viability of biodegradable polymers can be significantly enhanced by the incorporation of carbon nanotubes (CNTs). The environmental impact and persistence of these carbon nanotube/polymer nanocomposites (CNT/PNCs) after disposal will be strongly influenced by their microbial interactions, including their biodegradation rates. At the end of consumer use, CNT/PNCs will encounter diverse communities of microorganisms in landfills, surface waters, and wastewater treatment plants. To explore CNT/PNC biodegradation under realistic environmental conditions, the effect of multi-wall CNT (MWCNT) incorporation on the biodegradation of polyhydroxyalkanoates (PHA) was investigated using a mixed culture of microorganisms from wastewater. Relative to unfilled PHA (0% w/w), the MWCNT loading (0.5-10% w/w) had no statistically significant effect on the rate of PHA matrix biodegradation. Independent of the MWCNT loading, the extent of CNT/PNC mass remaining closely corresponded to the initial mass of CNTs in the matrix suggesting a lack of CNT release. CNT/PNC biodegradation was complete in approximately 20 days and resulted in the formation of a compressed CNT mat that retained the shape of the initial CNT/PNC. This study suggests that although CNTs have been shown to be cytotoxic towards a range of different microorganisms, this does not necessarily impact the biodegradation of the surrounding polymer matrix in mixed culture, particularly in situations where the polymer type and/or microbial population favor rapid polymer biodegradation.

Biodegradation of Poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) Plastic under Anaerobic Sludge and Aerobic Seawater Conditions: Gas Evolution and Microbial Diversity

Poly(3-hydroxybutyrate- co-3-hydroxyhexanoate) (poly(3HB- co-3HHx)) thermoplastics are a promising biodegradable alternative to traditional plastics for many consumer applications. Biodegradation measured by gaseous carbon loss of several types of poly(3HB- co-3HHx) plastic was investigated under anaerobic conditions and aerobic seawater environments. Under anaerobic conditions, the biodegradation levels of a manufactured sheet of poly(3HB- co-3HHx) and cellulose powder were not significantly different from one another over 85 days with 77.1 ± 6.1 and 62.9 ± 19.7% of the carbon converted to gas, respectively. However, the sheet of poly(3HB- co-3HHx) had significantly higher methane yield ( p ≤ 0.05), 483.8 ± 35.2 mL·g^-1 volatile solid (VS), compared to cellulose controls, 290.1 ± 92.7 mL·g^-1 VS, which is attributed to a greater total carbon content. Under aerobic seawater conditions (148-195 days at room temperature), poly(3HB- co-3HHx) sheets were statistically similar to cellulose for biodegradation as gaseous carbon loss (up to 83% loss in about 6 months), although the degradation rate was lower than that for cellulose. The microbial diversity was investigated in both experiments to explore the dominant bacteria associated with biodegradation of poly(3HB- co-3HHx) plastic. For poly(3HB- co-3HHx) treatments, Cloacamonales and Thermotogales were enriched under anaerobic sludge conditions, while Clostridiales, Gemmatales, Phycisphaerales, and Chlamydiales were the most enriched under aerobic seawater conditions.

Biodegradation of bioplastics in natural environments

The extensive production of conventional plastics and their use in different commercial applications poses a significant threat to both the fossil fuels sources and the environment. Alternatives called bioplastics evolved during development of renewable resources. Utilizing renewable resources like agricultural wastes (instead of petroleum sources) and their biodegradability in different environments enabled these polymers to be more easily acceptable than the conventional plastics. The biodegradability of bioplastics is highly affected by their physical and chemical structure. On the other hand, the environment in which they are located, plays a crucial role in their biodegradation. This review highlights the recent findings attributed to the biodegradation of bioplastics in various environments, environmental conditions, degree of biodegradation, including the identified bioplastic-degrading microorganisms from different microbial communities.

Microbial production of polyhydroxyalkanoates (PHAs) and its copolymers: A review of recent advancements

Traditional mineral oil based plastics are important commodity to enhance the comfort and quality of life but the accumulation of these plastics in the environment has become a major universal problem due to their low biodegradation. Solution to the plastic waste management includes incineration, recycling and landfill disposal methods. These processes are very time consuming and expensive. Biopolymers are important alternatives to the petroleum-based plastics due to environment friendly manufacturing processes, biodegradability and biocompatibility. Therefore use of novel biopolymers, such as polylactide, polysaccharides, aliphatic polyesters and polyhydroxyalkanoates is of interest. PHAs are biodegradable polyesters of hydroxyalkanoates (HA) produced from renewable resources by using microorganisms as intracellular carbon and energy storage compounds. Even though PHAs are promising candidate for biodegradable polymers, however, the production cost limit their application on an industrial scale. This article provides an overview of various substrates, microorganisms for the economical production of PHAs and its copolymers. Recent advances in PHAs to reduce the cost and to improve the performance of PHAs have also been discussed.

Comparative study on the production of poly(3-hydroxybutyrate) by thermophilic Chelatococcus daeguensis TAD1: a good candidate for large-scale production

In spite of numerous advantages on operating fermentation at elevated temperatures, very few thermophilic bacteria with polyhydroxyalkanoates (PHAs)-accumulating ability have yet been found in contrast to the tremendous mesophiles with the same ability. In this study, a thermophilic poly(3-hydroxybutyrate) (PHB)-accumulating bacteria (Chelatococcus daeguensis TAD1), isolated from the biofilm of a biotrickling filter used for NOx removal, was extensively investigated and compared to other PHB-accumulating bacteria. The results demonstrate that C. daeguensis TAD1 is a growth-associated PHB-accumulating bacterium without obvious nutrient limitation, which was capable of accumulating PHB up to 83.6 % of cell dry weight (CDW, w/w) within just 24 h at 45 °C from glucose. Surprisingly, the PHB production of C. daeguensis TAD1 exhibited strong tolerance to high heat stress as well as nitrogen loads compared to that of other PHB-accumulating bacterium, while the optimal PHB amount (3.44 ± 0.3 g l(-1)) occurred at 50 °C and C/N = 30 (molar) with glucose as the sole carbon source. In addition, C. daeguensis TAD1 could effectively utilize various cheap substrates (starch or glycerol) for PHB production without pre-hydrolyzed, particularly the glycerol, exhibiting the highest product yield (Y P/S, 0.26 g PHB per gram substrate used) as well as PHB content (80.4 % of CDW, w/w) compared to other carbon sources. Consequently, C. daeguensis TAD1 is a viable candidate for large-scale production of PHB via utilizing starch or glycerol as the raw materials.

On the heterogeneous composition of bacterial polyhydroxyalkanoate terpolymers

Poly(3-hydroxybutyrate-4-hydroxybutyrate-3-hydroxyvalerate) (P(3HB-4HB-3HV)) terpolymers of low 3-hydroxyvalerate (3HV) content (1.7-6.4%) with 4-hydroxybutyrate (4HB) molar fractions from 1.8% to 35.6% were produced by fed-batch cultivation of Cupriavidus necator DSM545. Waste glycerol, γ-butyrolactone and propionic acid were used as main carbon source, 4HB and 3HV precursors, respectively. Uniaxial tensile tests were performed on the corresponding biopolymers. The Young's modulus and tensile strength of P(3HB-4HB-3HV) decreased, whereas the elongation at break increased with the 4HB molar%, following the general trend described for poly(3-hydroxybutyrate-4-hydroxybutyrate) (P(3HB-4HB)) but with pronounced lower elasticity. Differential scanning calorimetry results indicate that the temperature of crystallization and enthalpy of melting decreased as the 4HB% increased. No crystallization was observed in terpolymers containing more than 30% of heteromonomers (4HB and 3HV) even though multiple melting events were detected. Terpolymer fractions of different composition were obtained by solvent-fractionation of the original bacterial terpolymers.

Recent advances in synthetic bioelastomers

This article reviews the degradability of chemically synthesized bioelastomers, mainly designed for soft tissue repair. These bioelastomers involve biodegradable polyurethanes, polyphosphazenes, linear and crosslinked poly(ether/ester)s, poly(ε-caprolactone) copolymers, poly(1,3-trimethylene carbonate) and their copolymers, poly(polyol sebacate)s, poly(diol-citrates) and poly(ester amide)s. The in vitro and in vivo degradation mechanisms and impact factors influencing degradation behaviors are discussed. In addition, the molecular designs, synthesis methods, structure properties, mechanical properties, biocompatibility and potential applications of these bioelastomers were also presented.

Production and characterization of bacterial polyhydroxyalkanoate copolymers and evaluation of their blends by fourier transform infrared spectroscopy and scanning electron microscopy

Rhizobium meliloti produced a copolymer of short chain length polyhydroxyalkanoate (scl-PHA) on sucrose and rice bran oil as carbon substrates. Recombinant Escherichia coli (JC7623ABC1J4), bearing PHA synthesis genes, was used to synthesize short chain length-co-medium chain length PHA (scl-co-mcl-PHA) on glucose and decanoic acid. Fourier transform infrared spectroscopy (FTIR) spectra of the PHAs indicated strong characteristic bands at 1282, 1723, and 2934 cm(-1) for scl-PHA and at 2933 and 2976 cm(-1) for scl-co-mcl-PHA polymer. Differentiation of polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-hydroxyvalerate-P(HB-co-HV) copolymer was obseverd using FTIR, with absorption bands at 1723 and 1281 for PHB, and at 1738, 1134, 1215 cm(-1) for HV-copolymer. The copolymers were analyzed by GC and (1)H NMR spectroscopy. Films of polymer blends of PHA produced by R. meliloti and recombinant E. coli were prepared using glycerol, polyethylene glycol, polyvinyl acetate, individually (1:1 ratio), to modify the mechanical properties of the films and these films were evaluated by FTIR and scanning electron microscopy.